Lists of 12

The Twelve Tribes of Israel

The Twelve Minor Prophets

They are called "minor" not because they are unimportant, but because their books are shorter than the "major" prophets: Isaiah, Jeremiah, and Ezekiel, and sometimes Daniel.

The Twelve Apostles

The twelve disciples of our Lord Jesus, whom he also named apostles: See Luke 6:12-16.

Note: Acts 1:15-25 tells how, after Judas Iscariot had committed suicide and our Lord had ascended into heaven, Matthias was chosen to replace Judas,  "and he was numbered with the eleven apostles."